ByThomas D'Evelyn, Thomas D'Evelyn is general editor for the humanities at Harvard University Press.November 27, 1989

FULL of tangy, crunchy morsels of history, fiction, and theory, Umberto Eco's new novel is like a salad bar. It's all good for you - and at 641 pages there's plenty of it; it boasts the occasional hot pepper of insight; and it may serve as an appetizer or to clean the palate after the roast beef and potatoes of more serious fare.

But what could be more serious than a serio-comic interpretation of the modern mind? Like Erasmus and Swift, Eco plays the fool to teach us better about ourselves. Unlike his best-selling ``The Name of the Rose,'' ``Foucault's Pendulum'' is short on mystery and atmosphere and suspense. On the other hand, it's packed with lore - alchemical, Rosicrucian, theosophist, et cetera (and in the tradition this novel is exploring, et cetera is like a magic spell!).

A professor of semiotics at the University of Bologna and a specialist on the Middle Ages, Eco is fascinated by the fascination certain medieval ideas still hold for us. The time is the present: The characters provide several points of view from which to explore our credulous age.

Eco's starting point in his interpretation of the modern world is suggested by one of the epigraphs attached to one of the 120 chapters. It's from Karl Popper: ``The conspiracy theory of society ... comes from abandoning God and then asking: `Who is in his place?'''

Put another way: Conspiracy theories abound when the meaning of history is in doubt. Is there a plot in the random events of history? The hero of the novel - he calls himself the Sam Spade of culture - begins to think so when he sets out to find a friend who has just phoned calling for help. They are after him. As it turns out, ``They'' are after the ``Plan.'' But it takes 600 pages of flashback to tell us why, what the plan may be, and whether it even exists.

The hero, whose name is Casaubon (after the dried-up scholar in ``Middlemarch'' and also after a Tudor debunker of alchemy), is a full-time graduate student writing a thesis on the Knights of the Temple. The Templars became so powerful in the late Middle Ages that they threatened the Pope, who had their leader executed as a heretic. Casaubon almost convinces himself that the plan derives from the Templar's secret knowledge of the future and how to gain world power.

The search for the plan involves the hero in scams and escapades. He becomes consultant to a publisher who exploits the popular weakness for occult connections; a volume on metals grows to include alchemy! He visits Brazil, falls in love, and toys with local mystical rites. This section includes a vivid and moving portrait, sometimes satirical, of the human cost of delusion.

The mind-set of the novel is '60s-ish: We witness some student demonstrations and our hero draws the inevitable parallel between the Knights Errant and the New Left. On a certain sunny afternoon in Milan - ``yellow facades and a softly metallic sky'' - he watches a student demonstration and thinks first of a modern painter, Dufy, then of a medieval one, Guillaume Dufay: ``I had the impression of being in a Flemish miniature. In the little crowds gathered on either side of the marchers, I glimpsed some androgynous women waiting for the great display of daring they had been promised.''

THE portrait of the friend Casaubon sets out to rescue may explain why the novel sold 600,000 copies in the first couple of months in Italy. Bilbo is 15 years older than Casaubon. He's rooted in war-torn Italy. He's also haunted by failure, a singular inability to seize the main chance. His pathetic but lyrical adoration of a tough Italian beauty who sometimes thinks she's Sophia - wisdom - incarnate, helps bind these pages together. In the end, Bilbo learns the wisdom of saying ``no.''

The heart of this book's mystery is not the plot but why human beings tend to invent plots when they could just live day by day. The human mind loves to make comparisons and draw analogies.

A major comparison controls most of the detail of this book: The modern world is in many ways like the early Middle Ages. ``A splendid epoch,'' Eco's narrator says early in the book, ``a time dazzled by ecstasies and peopled with presences, emanations, demons, and angelic hosts.'' Recall that some of the widely followed ``wisemen'' of our times, from Timothy Leary to Aldous Huxley, held some strange beliefs and generally flirted with ideas that the orthodox consider heretical. As Eco notes, his ``splendid epoch'' of hodgepodge paganism yielded to the iron law and iron fist of the church.

In any event, reading Eco one starts to hear echoes. The title, for example, refers to the pendulum one sees in science museums, the long cord that swings in an arc determined by the rotation of the earth. The pendulum suggests that there's a point outside one can depend on. The pendulum plays a grisly role in the wild and scary final scene.

The title also refers to another Foucault, a modern historian who has denied the objectivity of fact, who thinks history is myth. But this Foucault is never mentioned in the book. The title is an in-joke. Or is there an occult resemblance between the scientist and the historian?

The point is not to answer such questions but to not be too stuffy to explore them. Eco is in control. True, the novel can seem as interminable as a medieval romance, and, like his model Sam Spade, our hero is an incorrigible romantic. A faithful friend to Bilbo, he can't quite live up to the responsibilities of fatherhood.

He's also no match for ``Them,'' who, plan or no plan, are still on his trail at novel's end. But the book has a point. Its wisdom is contained in a two-sided coin: the courage to say ``no'' (to occult resemblances, to romantic notions of all sorts) on one side, and, on the other, the capacity to seize the occasion, live life to the hilt.

That these impulses may check each other and tongue-tie the would-be wiseman is also reflected in the curious reticences and careful shaping of what often seems like a very wordy book. Satire or salad bar, ``Foucault's Pendulum'' is a salubrious feast of words and ideas.